This invention relates to controlling a robot in real time to track along a path, and more particularly to a method of modifying the taught motion of a robot using path sensor information and to an improved robot having such a tracking control.
Nearly all commercial industrial robots employ the taught path method for programming desired motions. The robot operator moves the robot end effector to its desired location using a special purpose terminal known as a teach pendant. Once in position, the robot control computer is instructed to store the coordinates of the point. The desired path is programmed as a sequence of such stored points. Using the teach pendant, the operator may also specify the velocity along the path, as well as welding conditions or other parameters. Similarly, the programmed path may be downloaded from an external computer performing off-line robot motion planning. During normal operation or playback, the robot controller performs the necessary interpolation routines required to command the robot mechanicals to move in linear, circular or articulated motion.
The taught path can be varied while the robot is in the teach mode, but is limited to a single path in the playback or automatic mode. In many applications, it is desirable to modify the taught motion of the robot, to compensate for misplacement of the workpiece, account for dimensional tolerances, or account for movement of the workpiece due to thermal distortion. Present robot controls allow limited modification to the taught path motion which are restricted to fixed offsets for selected points. The offsets are limited to set delta x, y, z changes prior to playback of a given path program.
For those applications where path changes are wanted between workpiece setups or may vary during execution of the motion, the pre-taught program is not adequate and it becomes necessary to provide a means of deviating from the pre-taught path based on inputs from a real time path sensor. The deviation from the taught path has to be accomplished in coordination with the taught path of the robot. The motion of the conventional robot is accomplished as a series of incremental absolute position moves along a taught path at time intervals of 10 to 40 milliseconds. For real time steering, the relative motion of the robot end effector tip to the real path has to be updated at a frequency which will allow the robot to move along the real path without significant errors. There are many advantages to combining sensor-based control with taught path programming. The latter provides a convenient means of coordinating robot motions with those of peripheral devices, such as positioners, welders, spray painters, conveyors, etc. Robots programmed in this manner are readily connected to interlock systems essential to the safety of human operators and neighboring equipment. Second, use of taught path information as a reference control command guides the end effector close to the proper path. In so doing the amplitude of control corrections will be reduced, as will the operating range required of sensors. Third, the taught path information provides a valuable backup data base, to be used in case of a sensor or signal processing malfunction. The robot controller can rely on taught path data until valid sensor measurements are required.
Another tracking scheme is that sensor data may be used as the only information guiding the robot along the path, without reference to a taught path. An example of this method is given in U.S. Pat. No. 4,542,279, A. W. Case, Jr. et al, "Microvector Control for Edge and Joint Following".